1. Field of the Invention
The present invention generally relates to an infusion device for performing an infusion by repeatedly sucking/ejecting a liquid, in particular, to an infusion device for performing an infusion at a minimal flow rate used by an analysis device such as a liquid chromatography, etc. More particularly, the present invention relates to a check valve for preventing a reverse flow of a liquid.
2. Description of Related Art
In the liquid chromatography, a pressurized, accurate, and stable flow rate is required to transport a liquid as a mobile phase. Thus, a plunger type infusion device is mostly used.
FIG. 4 shows a schematic structure of an ordinary plunger type infusion device by taking an infusion device using a cam as a mechanism for moving a plunger back and forth as an example. In the plunger type infusion device, a cam shaft 22 of a cam 21 is rotated by being connected with a rotary shaft of a motor (not shown). A cam follower 23 is externally connected to the cam 21 and rotated along with the rotation of the cam 21. The cam follower 23 and a sliding follower shaft 24 are securely installed with a cylindrical crosshead 25. Thus, the rotation of the cam 21 is converted into a linear back-and-forth movement of the crosshead 25. The crosshead 25 normally applies a pressure to the right direction (the side of the cam 21) of the figure by a spring 27, and thus the cam 21 and the cam follower 23 normally remain a fastening state. A cylindrical plunger 28 extends to be installed on a shaft core of the crosshead 25, such that a volume of a working fluid chamber V may be increased or decreased periodically with the back-and-forth movement of the plunger 28. The plunger 28 is embedded in and inserted into an annular sealing ring 29 and then moved back and forth, thereby preventing the working fluid in the working fluid chamber V from leaking towards the crosshead 25.
In order to limit a flow direction of the transported working fluid to a direction only from bottom to top in FIG. 4, check valves 31a and 31b are inserted into an ejection opening and a suction opening of the working fluid chamber V.
When the plunger 28 is moved rightwards, such that the volume of the working fluid chamber V is increased, the working fluid is sucked into the working fluid chamber V through the check valve 31b at the side of the suction opening (at the bottom of the figure). At this time, the check valve 31a is used to prevent the suction of the working fluid which comes from the side of the ejection opening (at the top of the figure). Then, when the plunger 28 is moved leftwards, such that the volume of the working fluid chamber V is decreased, the working fluid is ejected from the working fluid chamber V through the check valve 31a at the ejection side. At this time, the check valve 31b is used to prevent the flow of the working fluid towards the side of the suction opening.
FIG. 4(b) shows a schematic structure of a check valve 31. A sphere 35 is contained in a space surrounded by a casing 32 and a valve seat 30. The sphere 35 moves with the flow of the liquid and is separated from or joined to the valve seat 30 (Patent Document 1, etc.). When the liquid flows in the arrow direction in FIG. 4(b), the sphere 35 is departed from the valve seat 30 to open a flow path. When the liquid tends to flow in an opposite direction to the arrow, the sphere 35 abuts against the valve seat 30 to block the flow path, thereby preventing the liquid from flowing in the opposite direction.
In recent years, in the field of liquid chromatography, there are higher requirements in performing an analysis at a minimal flow rate (tens of nL˜several μL/minute), and the infusion precision of the infusion device has become more important. In order to realize the infusion precision at the required flow rate, it is essential to ensure the tightness of the check valve. As described above, the check valve is a valve in which the sphere and the valve seat having the through hole are separated from or joined to each other according to the liquid flow. In order to improve the tightness between the sphere and the valve seat in an abutting state, a check valve being precisely processed and having strictly managed geometric tolerance is used. In FIG. 5, FIG. 5(a) is a sectional view taken along a central line of the valve seat 30 when the valve seat 30 and the sphere 35 are in the abutting state; FIG. 5(b) is an enlarged view of an abutting portion when the sphere 35 and the valve seat 30 are in the abutting state; and FIG. 5(c) is a plan view viewed from one side of the valve seat abutted against the sphere. The valve seat 30 has a through hole 30h penetrating there-through, and the transported liquid passes through the through hole 30h. The abutting portion of the valve seat 30 against the sphere 35 is processed to form a concave spherical surface having the same diameter as a diameter Φb of the sphere 35. Accordingly, when the flow path is blocked, the valve seat 30 in a shape of a circular ring having a width W contacts the sphere 35. The tightness is improved by performing a mirror surface processing on this contact surface S0.